72 
FLORIDA GEOLOGICAL SURVEY-15TH ANNUAL REPORT 
aluminous silicates, as feldspar. It is white in color, slightly plastic, has a 
hardness of 2-2.5 and a specific gravity of 2.2-2.6. 
Crystals of kaolinite are of very rare occurrence in clays but have 
been described by several writers. 2 According to Hickling 3 the kaolinite 
occurs in irregularly hexagonal prisms with rough faces which show 
strong transverse striations that correspond to the basal cleavage. 
There are several minerals very closely related to kaolinite. These 
are all hydrous aluminum silicates, but it is doubtful if all commonly 
placed in this class are really distinct species. They sometimes occur 
in crystalline form, but more frequently occur in the amorphous condi¬ 
tion! These minerals are halloysite, indianaite, pholerite, rectorite, new- 
tonite, allophane, cimolite, montmorillonite, pyrophyllite, collyrite, and 
schrotterite. Some of these minerals can be identified by their optical 
properties. 1 
There are many other minerals not related to kaolinite which are 
often found in clays and do not decompose readily. They may be briefly 
treated as follows: 
Quartz —Si02. This mineral is found in practically every clay, though 
usually in very fine grains and sometimes in very small quantities. It 
may also occur in the amorphous form. In residual clays the grains are 
usually angular while in the sedimentary clays they are somewhat 
rounded, due to the rolling and tossing about they have received by the 
water action. In quantity in clays quartz ranges from less than one per 
cent in some white sedimentary clays to more than eighty per cent in 
some other clays. Quartz fuses at 1830° C. (cone 35), 2 but in the 
presence of other minerals which act as a flux it may soften at a lower 
temperature. In clays it affects the fusibility, shrinkage, plasticity, and 
bonding strength, depending upon the amount and texture of the quartz 
present. 
2 Dick, M., Mining Magazine, Vol. VIII, p. 15, 1876. 
Reusch, H., lahrb. f. Min., Vol. II, p. 70, 1887. 
Johnson, S. W., and Blake, J. M., American Journal of Science, II, Vol. XLIII, p. 
351. 1867. 
3 Hickling, G., China Clay; Its Nature and Origin, Trans. Inst. Min. Eng. 
(England), Vol. 36, 1908-9. 
iLarsen, E. S., The Microscopic Determination of the Non-opaque Minerals, U. 
S. Geol. Survey Bull. 679, 1921. 
2 Ries, H., Clays, Their Occurrence, Properties and Uses, p. 55, 1908. 
